We are fans of 'alternative' web browsers sometimes. On Windows XP in particular, Internet Explorer 8 became rather cumbersome compared to alternative browsers such as Mozilla Firefox. In situations where Internet Explorer failed to load, we'd often suggest Firefox as an alternative.
Being honest about it though, since Windows 7 arrived (and Internet Explorer 9 comes with it), I can't think of any good reason one might want to install a different web browser at all. IE9 does the job perfectly. It's fast, reliable, and easy to 'reset' to factory defaults if users fluff it up by visiting dodgy web sites.
A new study published recently reveals that Microsoft Internet Explorer is one of the safest browsers to use and is significantly more effective at blocking Malware than other browsers.
I'd summarise by saying that Internet Explorer 9 is the ONLY browser your users should be allowed to use unless there is a good reason to have an alternative browser on their computer. If they really need a new browser then get one from www.opera.com. This browser has been around donkeys years longer that Google Chrome and in my experience is much faster too.
Remember though that Internet Explorer 10 is already out (we're using it on Windows 8) and we've had no problems so far.